Devices for generating electromagnetic radiation by means of a laser-produced plasma, such as droplet-based laser-produced plasma (LPP) light sources are known from the prior art. These devices are capable of producing very bright point sources of light over an extremely broad range of wavelengths from X-ray to visible light depending upon the application. These high brightness point sources are used for example in the semiconductor industry as well as other manufacturing industries within scanning systems for detecting defects during the semiconductor manufacturing process. There is also a need for these sources in advanced high-resolution microscopes for studies of cell biology or additive manufacturing.
Droplet-based LPP light sources work by generating a high temperature plasma, particularly within a vacuum chamber. Therein, particularly, a droplet train of fuel or target material is generated within a droplet dispenser. A positioning system directs the droplet train through a laser focus. As the droplets align with the laser focus a high energy laser pulse irradiates the droplet, evaporating and ionizing a portion of the target material generating a high temperature plasma. This plasma acts as almost as a point source of radiation. The wavelength and brightness of the light source depends on the choice of fuel and the energy of the laser pulse. For the generation of extreme ultraviolet light (EUV) at 13.5 nm the target material is typically pure tin, lithium or xenon.
In these sources debris from the exploding droplet (often liquid metal) remains a challenge, since the liquid splashes coat optics and nearby instrumentation within the vacuum chamber, making long term source operation challenging.
The unevaporated portion of a droplet typically starts as a spherical shape that when subjected to a shock wave from the expanded plasma produces splash fragments of a predetermined size, wherein the fragment size distribution is highly dependent on droplet and laser parameters. The larger these splashes are, the more difficult it is to protect the source optics.
According to the prior art, the size of debris particles can be reduced by applying a single pre laser pulse to the target, thereby shaping the target prior to the main laser pulse (US 2017/0027047 A1, U.S. Pat. No. 8,164,076 B2, US 2006/0215712 A1, U.S. Pat. No. 9,820,368 B2, U.S. Pat. No. 7,928,416 B2, U.S. Pat. No. 7,239,686 B2).
However, these pre-pulsing methods known from the prior art have the disadvantage that only a limited repertoire of target shapes which are sub-optimal in terms of debris mitigation, conversion efficiency and/or stability of operation can be obtained.